Netty-----初探

　　今天看gateway 实现的时候看到个哥们基于的netty实现的gateway。so，解析一下Netty。

废话少说，maven pom 引入，down 下jar包。看了下netty的包结构，还是挺明确的，不像spring包那么多。

buffer,channel，是包装的JDK 的nio Buffer,Channel等类。

在io.netty.buffer.ByteBuf找到了如下的注释，解释了 limit ,flip等原生JDK api 的含义。

 /**
  * A random and sequential accessible sequence of zero or more bytes (octets).
  * This interface provides an abstract view for one or more primitive byte
  * arrays ({@code byte[]}) and {@linkplain ByteBuffer NIO buffers}.
  *
  * <h3>Creation of a buffer</h3>
  *
  * It is recommended to create a new buffer using the helper methods in
  * {@link Unpooled} rather than calling an individual implementation's
  * constructor.
  *
  * <h3>Random Access Indexing</h3>
  *
  * Just like an ordinary primitive byte array, {@link ByteBuf} uses
  * <a href="http://en.wikipedia.org/wiki/Zero-based_numbering">zero-based indexing</a>.
  * It means the index of the first byte is always {@code 0} and the index of the last byte is
  * always {@link #capacity() capacity - 1}.  For example, to iterate all bytes of a buffer, you
  * can do the following, regardless of its internal implementation:
  *
  * <pre>
  * {@link ByteBuf} buffer = ...;
  * for (int i = 0; i &lt; buffer.capacity(); i ++) {
  *     byte b = buffer.getByte(i);
  *     System.out.println((char) b);
  * }
  * </pre>
  *
  * <h3>Sequential Access Indexing</h3>
  *
  * {@link ByteBuf} provides two pointer variables to support sequential
  * read and write operations - {@link #readerIndex() readerIndex} for a read
  * operation and {@link #writerIndex() writerIndex} for a write operation
  * respectively.  The following diagram shows how a buffer is segmented into
  * three areas by the two pointers:
  *
  * <pre>
  *      +-------------------+------------------+------------------+
  *      | discardable bytes |  readable bytes  |  writable bytes  |
  *      |                   |     (CONTENT)    |                  |
  *      +-------------------+------------------+------------------+
  *      |                   |                  |                  |
  *      0      <=      readerIndex   <=   writerIndex    <=    capacity
  * </pre>
  *
  * <h4>Readable bytes (the actual content)</h4>
  *
  * This segment is where the actual data is stored.  Any operation whose name
  * starts with {@code read} or {@code skip} will get or skip the data at the
  * current {@link #readerIndex() readerIndex} and increase it by the number of
  * read bytes.  If the argument of the read operation is also a
  * {@link ByteBuf} and no destination index is specified, the specified
  * buffer's {@link #writerIndex() writerIndex} is increased together.
  * <p>
  * If there's not enough content left, {@link IndexOutOfBoundsException} is
  * raised.  The default value of newly allocated, wrapped or copied buffer's
  * {@link #readerIndex() readerIndex} is {@code 0}.
  *
  * <pre>
  * // Iterates the readable bytes of a buffer.
  * {@link ByteBuf} buffer = ...;
  * while (buffer.readable()) {
  *     System.out.println(buffer.readByte());
  * }
  * </pre>
  *
  * <h4>Writable bytes</h4>
  *
  * This segment is a undefined space which needs to be filled.  Any operation
  * whose name ends with {@code write} will write the data at the current
  * {@link #writerIndex() writerIndex} and increase it by the number of written
  * bytes.  If the argument of the write operation is also a {@link ByteBuf},
  * and no source index is specified, the specified buffer's
  * {@link #readerIndex() readerIndex} is increased together.
  * <p>
  * If there's not enough writable bytes left, {@link IndexOutOfBoundsException}
  * is raised.  The default value of newly allocated buffer's
  * {@link #writerIndex() writerIndex} is {@code 0}.  The default value of
  * wrapped or copied buffer's {@link #writerIndex() writerIndex} is the
  * {@link #capacity() capacity} of the buffer.
  *
  * <pre>
  * // Fills the writable bytes of a buffer with random integers.
  * {@link ByteBuf} buffer = ...;
  * while (buffer.maxWritableBytes() >= 4) {
  *     buffer.writeInt(random.nextInt());
  * }
  * </pre>
  *
  * <h4>Discardable bytes</h4>
  *
  * This segment contains the bytes which were read already by a read operation.
  * Initially, the size of this segment is {@code 0}, but its size increases up
  * to the {@link #writerIndex() writerIndex} as read operations are executed.
  * The read bytes can be discarded by calling {@link #discardReadBytes()} to
  * reclaim unused area as depicted by the following diagram:
  *
  * <pre>
  *  BEFORE discardReadBytes()
  *
  *      +-------------------+------------------+------------------+
  *      | discardable bytes |  readable bytes  |  writable bytes  |
  *      +-------------------+------------------+------------------+
  *      |                   |                  |                  |
  *      0      <=      readerIndex   <=   writerIndex    <=    capacity
  *
  *
  *  AFTER discardReadBytes()
  *
  *      +------------------+--------------------------------------+
  *      |  readable bytes  |    writable bytes (got more space)   |
  *      +------------------+--------------------------------------+
  *      |                  |                                      |
  * readerIndex (0) <= writerIndex (decreased)        <=        capacity
  * </pre>
  *
  * Please note that there is no guarantee about the content of writable bytes
  * after calling {@link #discardReadBytes()}.  The writable bytes will not be
  * moved in most cases and could even be filled with completely different data
  * depending on the underlying buffer implementation.
  *
  * <h4>Clearing the buffer indexes</h4>
  *
  * You can set both {@link #readerIndex() readerIndex} and
  * {@link #writerIndex() writerIndex} to {@code 0} by calling {@link #clear()}.
  * It does not clear the buffer content (e.g. filling with {@code 0}) but just
  * clears the two pointers.  Please also note that the semantic of this
  * operation is different from {@link ByteBuffer#clear()}.
  *
  * <pre>
  *  BEFORE clear()
  *
  *      +-------------------+------------------+------------------+
  *      | discardable bytes |  readable bytes  |  writable bytes  |
  *      +-------------------+------------------+------------------+
  *      |                   |                  |                  |
  *      0      <=      readerIndex   <=   writerIndex    <=    capacity
  *
  *
  *  AFTER clear()
  *
  *      +---------------------------------------------------------+
  *      |             writable bytes (got more space)             |
  *      +---------------------------------------------------------+
  *      |                                                         |
  *      0 = readerIndex = writerIndex            <=            capacity
  * </pre>
  *
  * <h3>Search operations</h3>
  *
  * For simple single-byte searches, use {@link #indexOf(int, int, byte)} and {@link #bytesBefore(int, int, byte)}.
  * {@link #bytesBefore(byte)} is especially useful when you deal with a {@code NUL}-terminated string.
  * For complicated searches, use {@link #forEachByte(int, int, ByteBufProcessor)} with a {@link ByteBufProcessor}
  * implementation.
  *
  * <h3>Mark and reset</h3>
  *
  * There are two marker indexes in every buffer. One is for storing
  * {@link #readerIndex() readerIndex} and the other is for storing
  * {@link #writerIndex() writerIndex}.  You can always reposition one of the
  * two indexes by calling a reset method.  It works in a similar fashion to
  * the mark and reset methods in {@link InputStream} except that there's no
  * {@code readlimit}.
  *
  * <h3>Derived buffers</h3>
  *
  * You can create a view of an existing buffer by calling either
  * {@link #duplicate()}, {@link #slice()} or {@link #slice(int, int)}.
  * A derived buffer will have an independent {@link #readerIndex() readerIndex},
  * {@link #writerIndex() writerIndex} and marker indexes, while it shares
  * other internal data representation, just like a NIO buffer does.
  * <p>
  * In case a completely fresh copy of an existing buffer is required, please
  * call {@link #copy()} method instead.
  *
  * <h3>Conversion to existing JDK types</h3>
  *
  * <h4>Byte array</h4>
  *
  * If a {@link ByteBuf} is backed by a byte array (i.e. {@code byte[]}),
  * you can access it directly via the {@link #array()} method.  To determine
  * if a buffer is backed by a byte array, {@link #hasArray()} should be used.
  *
  * <h4>NIO Buffers</h4>
  *
  * If a {@link ByteBuf} can be converted into an NIO {@link ByteBuffer} which shares its
  * content (i.e. view buffer), you can get it via the {@link #nioBuffer()} method.  To determine
  * if a buffer can be converted into an NIO buffer, use {@link #nioBufferCount()}.
  *
  * <h4>Strings</h4>
  *
  * Various {@link #toString(Charset)} methods convert a {@link ByteBuf}
  * into a {@link String}.  Please note that {@link #toString()} is not a
  * conversion method.
  *
  * <h4>I/O Streams</h4>
  *
  * Please refer to {@link ByteBufInputStream} and
  * {@link ByteBufOutputStream}.
  */